varnish overhead when only passing to backend without caching

[Admin Beckspaced]

hello again,

my current setup is varnish 5 in front of apache web server with 30 
different domains / websites
in my vlc i can switch between sites and decide to do varnish caching or 
just passing to the backend with out caching.

what's the overhead / delay with varnish when i just pass everything to 
the backend and back to the client without varnish doing any caching?

i mean there must be some overhead as varnish is in between ...
... but how big is it? time delay?

does anyone here have any experience with this?

thanks & greetings
becki